Frequency modulating device



Feb. 22, L, DE FOREST FREQUENCY- MODULATING DEVI CE Filed Jan. 25, 1945 www,

Osc/L L 4 TOR 9 )100 llhllvfw# HUD/o @MPL/Flew /91 IN VE/vroe Les 05 Foesfr Patented Feb. 22, 1949 UNITED STATES PATENT OFFICE FREQUENCY MODULATING DEVICE.

Lee de Forest, Los Angeles, Calif.

Application January 25, 1945, Serial No. 574,516

7 Claims.

My invention relates to a method of and apparatus for modulating electric currents, particularly high frequency currents, such as those employed in radio communication equipment or other electrical transmission circuits, for the purpose of imparting to such currents various signal characteristics or voice modulations preparatory to the transmission of such information.

The invention specifically includes a type of electrically variable capacitor whose utility is not necessarily limited to the aforesaid arts. For the purpose of this disclosure I elect to describe a method of frequency modulation but those skilled in the art will readily recognize that the novel principles embodied in my invention may be usefully employed for effecting various kinds of modulation of electric currents and for performing various other functions.

The general object of my invention is to provide an electrically variable capacitor, whose capacitance may be modulated by voltages or currents, particularly those of audio frequency.

Another object is to provide a method of, and means for, producing broad band frequency modlation without employing complicated conventional circuits.

An important object of my invention is to provide an electronic capacitor whose Variable capacitance is modulated by alterations in the nature of the medium disposed between the spaced condenser plates.

A further object is to provide an electronic capacitor whose variable capacitance is modulated by tlie degree of ionization of a gas disposed between the spaced condenser plates.

An important feature of my modulating system is a cathode beam which modulates the capacitance of my capacitor. This allows a very great simplification in frequency modulation transmitter circuits by eliminating the necessity for' the various multiplier stages required in conventional modulation equipment.

Another feature of my modulating system is the modulation of the instantaneous capacitance of a condenser whose internal insulation engages an ionized gas.

Other objects and advantages of my invention, as well as a clear understanding of the principles involved therein and practical methods of applying them, will be suggested by the detailed description and claims to follow, taken with the accompanying drawings, in which:

Fig. 1 is a diagrammatical representation of one embodiment of my modulation system as 2 applied to an electrical circuit for the generation of high frequency currents.

Fig. 2 is a transverse sectional view of a tubular electrode, hereinafter referred to as an electron lens, employed for the purposes of the invenion.

Fig. 3 is a side elevational View as partially sectioned of the structure shown in plan on the right side of Fig. 1 exemplifying an embodiment employing some of the elements of a cathode-ray tube for the purposes of the invention.

Fig. 4 is a sectional view of the protected condenser plates employed in my electrically variable capacitor.

Figs. 5 and 6 illustrate diagrammatically the elements involved as parameters in an analytical determination of the electrical characteristics of my electrically variable capacitor: Fig. 5 shows a region between spaced layers of dielectric which enclose condenser plates and Fig, 6 indicates symbolically the variable conductance of such aregion.

Fig. 7 represents a detailed side view of one form of my electrically Variable capacitor and illustrates one of the operating principles involved therein.

Fig. 8 is a diagrammatic View of another embodiment of my modulation system including an electrically variable capacitor which is not confined within a cathode-ray tube.

The schematic arrangement shown in Figure 1 illustrates a modulation system, as one embodiment of the invention, whose principal components include a radiating circuit I0 for emitting electromagnetic waves produced by an undulatory electrical current whose intensity is increased by a power amplifier tube 20. These electrical undulations are generated in an oscillatory circuit 30 whose resonant frequency is modulated by a capacitor 40 whose capacitance is modulated by a cathode-ray whose deflection is modulated by a control circuit 50.

The radiating circuit I0, which includes an antenna or radiating dipole I2--I3 in series with the primary I4 of a radio frequency transformer I5, is inductively coupled by means of the seccondary I6 to the output circuit of the power amplifier vacuum tube 20. This output tube has an anode I'I whose electric potentia1 may be supplied by the positive terminal of a battery I8, or other suitable means, whose negative terminal is grounded to the cathode I9. These terminals of the source of direct current and voltage are shunted by a by pass condenser 2| which provides a low impedance path for radio frequency currents.

The direct current and voltage supplied by the battery i8 is conducted through the secondary I6 of the radio frequency transformer l5 and delivered to the anode il of the power amplier tube 2t, as well as any additional amplifier tubes, since it will be understood that additional amplification may frequently be desirable. Y

Such supplementary amplifying tubes would be interposed, of course, between the grid 22 of the power amplier tube 26 and the output of the master oscillator tube '23, whose-anode 2li-'may also receive electric potential from the anode through an intervening voltage-dropping resistor and a high frequency choke-coil 26.

The grid 22 of the power amplifier tube 20 is energized by the anode 24, unless intermediate stages of amplication are introduced. The bias ing potential-of the grid 22 is controlledby the voltage across a grid-leakresistor 2l, which-is grounded by connection with the cathode |9, This grid bias is also protected against the direct voltage vof the anode `24 by means of a grid -condenser 28.

The'modulated carrier wave, whose ampliii'cation has been described above, is generated by the master oscillator tube 23and the associated resonant circuitt whose high frequency oscillations are modulated by my electrically variable capacitor'4l.

This resonant circuit 30 includes a Variable capacitance 3| connected in parallel with an inductance'SZ which in this instance is illustrated in the form of an arrangement known as a butterfly circuit, which is characterized by la very high circuit Q and by the'particular combination oi the inductance loop 32 and a subtended stator 33, which encompass a rotor 34, The resonant frequency of this oscillatory circuit is dependent, also, upon the adjustment of the capacitance 3| and upon the Velectrically variable magnitude of the capacitance of the modulating capacitor'll), which'is connected in parallel with the capacitance 3 l This resonant circuit 36 energizes algridv35 of the oscillator tube 23, whose cathode 36 is grounded -to the negative terminal of the'battery i8. The biasingfpotential of the grid is provided by the voltage lacross a grid-leak resistor 3l, which is grounded by connection with the cathode 36. The variations of potential on the gridfrare amplified, of course, by theA tube 23, which delivers a modulated voltage signal at the: anode 24 to the grid condenser 28 of the amplifier tube 2c. This modulated signal voltage is also applied to the resonant circuit 30, Vsince oscillations are thus sustained. Accordingly, the anode '24 is connected to the stator 33, to the indu'ctance loop 32 and, by means of a sliding contact 38, to the rotor 34. A grid condenser 39 protects the grid 35 from the direct potential of the anode 24. This potential is thus applied to both of the condenser plates, or electrodes, 4| and 42 ofthe modulating capacitor 0.

It is noteworthy that the electrical circuit so far described in detail is electrically complete and self-suicient for generating' and radiating electromagnetic waves whose modulations are provided'by the variations of the resonant frequency of the oscillatory circuit 3|). The instantaneous Avalue of this resonant frequency is automatically controlled bythe-instantaneous lvalues of the conductance and capacitance of the capacitor 40,

parameters which are varied by the control circuit 5l).

The modulating capacitor 40 may be designed and employed in a variety of ways within the scope of my invention.

A cathode ray is directed between or below the electrodes 4l and 42 which form the condenser plates of my electrically variable capacitor 4U, as illustrated in the plan viewat the right in Fig. 1 or by the longitudinalside view'in Fig. 3. This cathode ray is emitted by an electron gun having a cathode 43, a tubular grid structure 44, a primary-anodeor-electron lens 45 and a secondary anode or collector plate 46 which serves as a target for the cathode-ray. The direct current lpotential required by the anodes c5 and 46 may be supplied by conventional means, such as a battery 41, whose negative terminal is connected to the cathode 43. Such potentials may be adjusted by appropriate series resistance, as indicated by the numeral 48 interposed betweent'he anode 46 and the positive terminal of the battery 41.

The modulation -of the capacitance and 'conductance of my electronic capacitoris .accomplished vby modulating-.the cathode'ray. -Fonthis purpose I prefer toremploy amagnetic field, directed approximately perpendicular to both the cathode ray `and thecondenser plates 4| and`42, suchas that produced by electromagnets 5| -and 52 connected in series aiding relationship and energized byan electrical currentin the control circuit 50. The terminals of the directly coupled 'windings of the electromagnets 5| and 52fmay'be connected into any control circuit Vwhich proof modulatingY electromotive-force, which may-be measured, studied,=analyzed or utilized by'means of my cathode-ray capacitor, linclude photo-electric cells, X-rays, radio tubes and television. l Such modulating electromotive forces-may also be employed by substituting an lelectrostatic -eld Yfor the electromagnetic eld described above, provided that vthe electrostatic eld is directedapproximately perpendicular tothe cathode ray and parallel to thefcondenser plates-4I and 42,;-asby means of parallel plates whose potential difference is supplied by the control-circuit'50,

`The performanceof my Ucathode-ray capacitor will be described and analyzed with'the 'aid of Figs-2, 3, 4, 5, 6, and 7.

Fig. 2 illustrates a desirableforrn'for the cross section of tubular electron lens, orprirnaryanode 45, This 'oval transverse sectionv has a longitudinal aXis A-A which is Aoriented substantially parallel to the condenser plates ll-and '42 and approximately perpendicularv to the direct-ionof the cathode'ray. The potential of this-primary anode 45 produces an electrostatic eld which tends to concentrate the cathode rayinto aswordshaped beam whose larger transverse dimension is parallel Vto the axis A--A.

As illustrated in Figs. 3 and 7 the direction O-B of this sword-shaped beam may be-modulatedby the aforesaid deflecting forces, suchas those supplied by the magnetic iield produced vby the electromagnets 5l and 52 whose axial'p'oles-are indicated bythe reference numeral A53. AThis modulation ofv the 'direction of the cathode ray may be Vvisualized'as avariationdn the vertical deflection of thesword-s'haped fbeam, as -`indicated by the deflection from the position O-B to the position -C in Fig. 7. The beam is thus caused to swing upwardly, to a greater or lesser degree, into the region between the condenser plates 4| and 42. This deflection is linearly proportional to the intensity of the control current in the electromagnets 5| and 52. In the absence of such modulation, the upper edge of the undeflected cathode beam is directed between or below the lower extremities 54 of the electrodes 4| and 42, approximately along the longitudinal axis of the modified cathode-ray tube. As illustrated in Figs. 3 and 4, the condenser plates 4| and 42 of my cathode-ray capacitor 4U are preferably enclosed in glass or other-suitable insulating material 55 in order to shieldthem from approaching ions or electrons. The electrical conductors which travel from these plates through the tubular glass shell, as at 56 and 51, in order to provide terminals for electrical connection with an external circuit, also should be encased in suitable insulating material, such as glass or an appropriate thermoplastic substance. These insulated electrical connectors may serve, of course, as supports for the insulated plates 4| and 42, either as illustrated in Fig. 3 or by extending the arms 58 and 59 to engage the opposite sides of the tubular vessel 60.

l My cathode-ray capacitor should be incorporated in a tubular vessel Ell which has been highly exhausted of air but may preferably contain a small amount of one of the noble gases, such as neon or argon. This rareiied gas will be ionized by the impact of collision between the electron beam and the gas molecules. The resulting positive ions are too heavy to be deflected very far by the electrostatic or magnetic fields. Consequently, many of these positive ions receive momentum from electron bombardment by the cathode ray which directs them toward the collector plate 4E. The remainder of these heavy positive ions drift toward :the negative cathode. In the absence of modulation, therefore, an equilibrium distribution of positive ions exists throughout the tubular vessel 6U; and the region D--E between the condenser plates 4| and 42 is occupied by a relatively few ions, which are drifting away from the high direct current potential of these plates toward the negative cathode. Such absence of ions in the region D-E indicates the existence of a dielectric of rareed gas whose dielectric constant partially determines the variable capaci- `tance of the capacitor 40.

The magnitude of this variable capacitance is also dependent upon the presence of a distribution of electrons on the outer surface of the insulation 55. These electron layers 6| and 62 are supported by the electrostatic force of attraction toward the charged condenser plates 4| and 42. Accordingly, my cathode-ray capacitor provides -three subordinate condensers C1, C and C2, electrically co-acting in series relationship, respectively comprised of the following pairs of conductive elements: 4|-||, 6|-52 and 62-42, where the elements 6| and 62 each represent the conductive layer of ions-electrons residing upon the outer surface of the insulation 55, as indicated in Figs. 4 and 5.

The capacitance of the pair of conductive elements 4|-5l or 62-42 is approximately represented by the conventional formula where D represents the distance between two parallel conducting planes whose area is A separated by electrical insulation whose dielectric constant 4is li. This constant is of the order of `7 to 9 for 75 glass, as in condenser C1 or C2, and approximately unity for the un-ionized gas dielectric in condenser C. The distance D is relatively smaller for C1 and Cz, perhaps al: inch, for example, as compared with 1A; to 1A inch between the elements 6| and 62. Hence the ratio Ci/C or Cz/C is of the order of 100, since either of the plates 4| or 42 is shielded on each side by a layer of ionselectrons, thus doubling the effective area.

The total capacitance of the series combination of condensers C1, C and C2 is approximately represented by since C1=C2 if the thickness of the insulation 55 is uniform, if the condenser plates 4| and 42 have the same area, etc., as illustrated in Figs. 4 and 5.

The foregoing description of the state of equilibrium existing in the unmodulated cathode-ray capacitor may be extended by considering the leakage conductance through the condenser C, as provided by the transverse conductance of the cathode ray whenever it is deflected from the normal position O-B to O-C, as indicated in Fig. "I, or from F toward D, as identified in Fig. 4. This leakage conductance is associated with the indicated variable resistance R identied in Fig. 6 by the numeral 65.

The elements of the cathode-ray capacitor, represented schematically in Fig. 6, now include the series arrangement of condensers Ci, C and C2 and the resistance R connected in parallel with the capacitance C. The impedance of this series-parallel arrangement will be radically altered whenever the sword-shaped cathode beam is deiiected or modulated. For example, if the beam is deected upwards, the region DE will become highly conductive as a result of the positive ions produced by the bombardment of gas molecules by the deflected beam. Alternatively, if the beam is deflected downwardly, the region DE will become electrically non-conductive as the positive ions are repelled from this region lg the potential of the condenser plates 4| and The cathode-ray capacitor will provide more effective modulation if the capacitance C1 is designed relatively larger than the capacitance C, e. g., the insulation 55 should have a minimum thickness, etc.

The effectiveness of the cathode-ray capacitor for modulation purposes may be approximately evaluated. For example, suppose that C1 has a value of 99 micro microfarads and C has a value of one micro microfarad; then the total impedance of the cathode-ray capacitor will be decreased one hundred fold whenever the cathoderay is deflected into the region DE between the condenser plates 4| and 42.

Another concept may be introduced by considering the cathode-ray capacitor as representing a simple condenser shunted by a resistance R. The impedance of this combination may be written ai ofthe layer 'of partially ionized gas lying between. the condenser plates fil and- 42.

Equation 3 may be approximately expressedV in the form where o is the conductance per unit of ionized area ai, depending on the nature of the gas, the degree of its ionization and the linverse thickness of. the layer of partially ionized gas between the condenser plates di and 42. The angular velocity w--Zvrj if j symbolizes the frequency of the oscillatory current.4 With a given voltage and cathode temperature, the tubular vessel 6G will contain a gas whose degree of ionization is constant.

The ionized area ai is a variable determined by the deflection of the ionizing cathode beam. The ionized area ai is therefore proportional to the intensity of the current inthe control circuit 58, if the electromagnets I and 52 are employed, or, alternatively, proportional to the audio frequency potential diiierence impressed on the deilecting plates of the tube by the control circuit 5t if an electrostatic field is employed instead oi a magnetic field for deecting the cathode beam. The total impedance of the .cathode ray capacitor is therefore inversely proportional to the intensity of the current in the control circuit or inversely j proportional to the potential difference impressed on the deecting plates by the control circuit. An approximately linear relationship exists, therefore, between the modulating force and the resulting modulation.

Alternatively, if the cross section of the cathode beam normally fills the inter-electrode region DEF, this cross section may be modulated by the control circuit ii in order to modulate the ionized area ci. Various means of reducing this cross section may be employed. For example, the potential of the primary anode or the control grid di; may be modulated by the Control circuit Eil. In this case. the total impedance of the cathode-ray capacitor is increased, rather than decreased, in proportion to the applied voltage.

Another alternative embodiment of my cathode-ray capacitor is illustrated in Fig. 8, which shows a tubular vessel it containing a rareed gas and having a constriction which may be flat- .7.;

tened, if desired, and partially surrounded by two external condenser plates 'il and i2 for the purpose of modulating the current in a transmission circuit l0@ such as that indicated schematically by the block diagram.

The. cathode beam, which modulates the capacitance of this cathode-ray capacitor, is generated by a cathode i3, represented by a coiled `filament, and concentrated by a control electrode or grid 'is and directed through a constricted portion 'i5 of the tubular vessel 'lil toward an anode or collector plate 56 whose-potential is supplied through a high frequency choke coil 'i1 and a series resistance control 'i8 by a suitable voltage supply such as the positive terminal off a batteri7 I.

The transmission circuit ino is indicated in Fig. 8 by a block diagram labeled Frequency modulated (F. M.) oscillator lili whose resonant frequency is determined by the variable capacitance of condenser plates 'H and 12. This oscillator it!! delivers a frequency modulated current to a radiating dipole Idil-m3.

A The capacitance of the cathode ray capacitor is modulated bythe degree of ionization of the rareed gas in the tubular vessel lil. Thiedegree. of `ionization may be modulated vin several Ways; two methods will be described; the,.rst method utilizes a grid control circuit. 80, shown. on the left side of Fig. 8; the second method utilizes a plate control circuit shownon the lower right side of Fig. 8.

The grid control circuit is connected betweenv the cathode i3 and the control grid 1.4. The audio frequency potential difference between the cathode i3 and the grid 74 is provided by-an audio amplifier 8| whose input circuit includes a microphone 82. The direct current potential difference between the cathode 'i3 and the. grid lli is provided by a battery 8.3 connected in parallel with a variable resistance 813. This potentiometer arrangement provides a negative vbiasing potential to the grid '14, which is protected from the direct current potential of the output circuit of the amplier 8l by interposinga grid condenser in series with the grid T14.

In the foregoing method, the degree of ionization ofthe rarefied gas in the tubular vessel 10 is .modulated by the variable potential of the control grid lf3.

In the following alternative method, the potential of the plate 1.5 is employed for modulating the degree of ionization of therareied gas. This potential is controlled by a so-called Helsing circuit l, comprising an audio amplier triodeztube I, connected in parallel with the ionized gas tube le, and an audio amplier Q2 whose input circuit includes Va microphone 93. lThe anode potential of the modulator tube si may be supplied by the plate 5%. The cathode of the tube 9| is connected to the negative terminal of the battery 179.

The modulation from the microphone S3 is amplified by the amplifier 92 and delivered to the cathode and grid of the triode 9| whose anode controls the audio frequency potential of the plate 'it of the ionization tube 7G.

The capacitance of the cathode-ray capacitor is thus modulated by the control circuit S0, which rnodulates'the degree of ionization of the rareed gas in the ionization tube l0.

Additional methods of controlling the Yioniza tion of the gas and thus modulating the capacitance of the cathode-ray capacitor are suggested by the foregoing principles and concepts.

For example, I .may employ a voice modulated coaxial magnetic ield for directly controlling the space-charge density of the cathode beam. This may be accomplished by positioning the longitudinal axis oi a solenoid substantially coincident with the cathode ray and energizing the solenoid with the voice modulated control current,1 1in order to modulate the capacitance ofthe cathoderay capacitor.

As a second example, I may employ a magnetic eld directed through the constricted portion "l5 of the ionization tube 10, i. e., directed perpendicular to the cathode beam therein, andmodulatev the intensity of this transverse magnetic i-leld by means of an amplier which receives voice modulations from a microphone kland deliversa modulated control current to the electromagnet's which provide the aforesaid transverse magnetic eld. The space charge density of the cathode beam would thus be modulated and the Capacitance of the cathode-ray capacitor correspondingly modulated.

By any of the various methods disclosed herein it will be seen that I am able to directly control the instantaneous frequency of amodulated carrier wave and to elect broad band" frequency 9 modulation without the interposition of complicated circuits comprising several multiplier stages.

Other adaptations of the invention may be employed for various purposes other than frequency modulation. Since the preferred form of my invention, which is described herein in specific detail for the purpose of disclosure and to illustrate the principles involved, will suggest to those skilled in the art various changes, modifications and substitutions that do not depart from my underlying concepts, I reserve the right to all such changes, modications and substitutions that properly come within the scope of my invention.

I claim as my invention:

1. In a variable capacitor, the combination of: a pair of spaced electrodes; a dielectric material between said electrodes; an ionizable dielectric medium between said electrodes; means for directing a stream of electrons through said ionizable medium to effect ionization thereof; and means for deilecting said stream of electrons relative to said electrodes to Vary the extent to which it is interposed between said electrodes whereby to vary the degree of ionization of the ionizable medium between said electrodes.

2. In a variable capacitor, the combination of: a pair of spaced electrodes; means for directing an electron beam through the space between said electrodes; and means for defiecting said electron beam relative to said electrodes to vary the amount of the space between said electrodes which is occupied thereby.

3. In a variable capacitor, the combination of: a pair of spaced electrodes; an ionizable medium in the space between said electrodes; means for directing Aan electron beam through the space between said electrodes to ionize said medium; and means for deilecting said electron beam relative to said electrodes to Vary the amount of said ionizable medium between said electrodes which is ionized thereby.

4. In a variable capacitor, the combination of: a pair of spaced electrodes; means for generating a stream of electrons; means for directing said stream of electrons through the space between said electrodes in a fan-shaped beam lying substantially in a plane intermediate said electrodes; and means for deilecting said electron beam in said plain to vary the amount of the space between said electrodes which is occupied thereby.

5. In a variable capacitor, the combination of: a pair of spaced, parallel electrodes; means for normally directing an electron beam parallel to said electrodes adjacent one edge thereof; and means for defiecting said electron beam into the space between said electrodes to vary the capacitance of said capacitor.

6. In a variable capacitor, the combination of: a pair of spaced electrodes; a dielectric material enclosing at least one of said electrodes; means for directing a stream of electrons through the space between said electrodes; and means for deiiecting said stream of electrons in a direction transverse to a reference line extending between said electrodes so as to vary the dielectric characteristics of the space between said electrodes.

7. In a variable capacitor, the combination of: a pair of spaced electrodes; a dielectric material enclosing at least one of said electrodes; an ionizable dielectric medium between said electrodes; means for directing a stream of electrons through said medium to effect ionization thereof; and means for defiecting said stream of electrons in a direction transverse to a reference line extending between said electrodes so as to vary the degree of ionization of said ionizable medium in the space between said electrodes, whereby to vary the capacitance of said capacitor.

LEE DEFOREST.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,716,161 Allcutt June 4, 1929 2,012,710 Crosby Aug. 27, 1935 2,032,620 Langmuir Mar. 3, 1936 2,193,710 Burnham Mar. 12, 1940 2,195,098 Skellett Mar. 26, 1940 2,241,976 Blewett et al May 13, 1941 2,243,829 Brett et al. June 3, 1941 2,278,690 Clarke Apr. 7, 1942 2,306,555 Mueller Dec. 29, 1942 2,404,098 Schade July 16, 1946 

